1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to a transmissive-type organic electroluminescent display device and a fabricating method of the same.
2. Discussion of the Related Art
Among flat panel displays, liquid crystal display (LCD) devices have been commonly used due to their thin profile, light weight, and low power consumption. However, the LCD devices are not self-luminescent and thus have low brightness. Further, LCD devices typically have a low contrast ratio and narrow viewing angle. Furthermore, LCD devices can have large overall size because of the need to use a backlight.
Organic electroluminescent display (OELD) devices have wide viewing angles and excellent contrast ratios because of their self-luminescence. Since the OELD devices do not require additional light sources, such as a backlight, the OELD devices have relatively small size, are light weight, and have low power consumption, as compared to LCD devices. OELD devices can be driven by low voltage direct current (DC) and have short response times on the order of microseconds. Since the OELD devices are solid phase devices, OELD devices can sufficiently withstand external impacts and have greater operational temperature ranges. In addition, the OELD devices can be manufactured at low cost since only deposition and encapsulation apparatus are necessary for manufacturing the OELD devices, thereby simplifying manufacturing processes.
OELD devices can be categorized as passive matrix-type OELD devices or active matrix-type OELD devices depending upon the method of driving the devices. The passive matrix-type OELD devices are commonly used due to their simplicity and ease of fabrication. However, the passive matrix-type OELD devices have scanning lines and signal lines that perpendicularly cross each other in a matrix configuration. Since a scanning voltage is sequentially supplied to the scanning lines to operate each pixel, an instantaneous brightness of each pixel during a selection period should reach a value resulting from multiplying an average brightness by the number of the scanning lines to obtain a required average brightness. Accordingly, as the number of the scanning lines increases, the applied voltage and current also increase. Thus, the passive matrix-type OELD devices are not adequate for high resolution and large-sized display since the device easily deteriorates during use and the power consumption is high.
Since the passive matrix-type OELD devices have many disadvantages, such as low image resolution, high power consumption and short operational lifetime, the active matrix-type OELD device has been developed to produce high resolution images in large display area displays. In the active matrix-type OELD devices, thin film transistors (TFTs) are disposed at each sub-pixel for use as a switching element to turn each sub-pixel ON and OFF. A first electrode in the sub-pixel connected to the TFT is turned ON/OFF while a second electrode facing the first electrode functions as a common electrode. In addition, a voltage supplied to the sub-pixel is stored in a storage capacitor, thereby maintaining the voltage for driving the device until a voltage of next frame is supplied, regardless of the number of the scanning lines. As a result, since an equivalent brightness is obtained with a low applied current, an active matrix-type OELD device has low power consumption and high image resolution over a large area.
FIG. 1 is a schematic circuit diagram of a pixel structure of an active matrix-type OELD device according to the related art. In FIG. 1, a scanning line 1 is arranged along a first direction, and a signal line 2 and a power supply line 3 that are spaced apart from each other are arranged along a second direction perpendicular to the first direction. The signal line 2 and the power supply line 3 crosse the scanning line 1 to thereby define a pixel area. A switching thin film transistor (TFT) TS, as an addressing element, is connected to the scanning line 1 and the signal line 2. A storage capacitor CST is connected to the switching TFT TS and the power supply line 3. A driving thin film transistor (TFT) TD, as a current source element, is connected to the storage capacitor CST and the power supply line 3. An organic electroluminescent (EL) diode DEL is connected to the driving TFT TD.
When a forward current is supplied to the organic EL diode DEL, an electron and a hole are recombined to generate electron-hole pairs at the PN junction of the organic EL diode DEL between an anode, which provides holes, and a cathode, which provides electrons. There is an energy difference between the electron-hole pair and the separated electron and hole. More particularly, the electron-hole pair has a lower energy than the separated electron and hole. The energy difference generates the emission of light.
The OELD devices are commonly categorized as top emission-type and bottom emission-type according to the direction of the emitted light. In the bottom emission-type OELD device, light is emitted from the bottom of the device through a bottom substrate including thin film transistors. However, the emitted light does not pass through electrode lines and the thin film transistors on the array substrate. Therefore, the aperture ratio of the bottom emission-type OELD device depends on the structure of the thin film transistors and the electrode lines. It is not easy to design the thin film transistors and electrode lines that minimally block light. In the top emission-type OELD device, light is emitted out the top of the device through a top substrate. The emission area for the top substrate of the top emission-type OELD device can reach about 70% to about 80% of the whole panel size. However, the air easily infiltrates the organic electroluminescent layer in the top emission-type OELD device.
To solve the problems of the top emission-type and the bottom emission-type OELD devices, which each have only one emission direction, an OELD device that emits light in both the upward and downward directions has been recently developed. FIG. 2 is a cross-sectional view of such a transmissive-type organic electroluminescent display (OELD) device according to the related art. As shown in FIG. 2, a gate electrode 12 is formed on a substrate 10 in a sub-pixel region Psub, which are the minimum constituent unit for a displayed image. A gate insulating layer 14 is formed over the entire surface of the substrate 10 and covering the gate electrode 12. A semiconductor layer 16 is formed on the gate insulating layer 14 and over the gate electrode 12. A source electrode 18 and a drain electrode 20 are formed on the semiconductor layer 16 and spacing apart from each other. A power supply line 22 is formed on the gate insulating layer 14 and connected to the source electrode 18.
A first passivation layer 26 is formed over the entire surface of the substrate 10 that includes the source electrode 18, the drain electrode 20 and the power supply line 22. The first passivation layer 26 includes a drain contact hole 24 for exposing the drain electrode 20. A first electrode 28 is formed on the first passivation layer 26 and is connected to the drain electrode 20 through the drain contact hole 24. An organic electroluminescent layer 30 and a second electrode 32 are sequentially formed over the entire surface of the substrate 10 to cover the first electrode 28. A second passivation layer 34 is formed on the second electrode 32. The second passivation layer 34 protects both the second electrode 32 and the organic electroluminescent display device from external impacts, moisture and the like.
The first electrode 28, the second electrode 32 and the organic electroluminescent layer 30 interposed between the first and second electrodes 28 and 32 constitute an organic electroluminescent (EL) diode. The gate electrode 12, the semiconductor layer 16, the source electrode 18 and the drain electrode 20 constitute a driving thin film transistor (TFT), which supplies currents to the organic EL diode. Although not shown in FIG. 2, a storage capacitor is connected to the power supply line 22. Further, the gate electrode 12 of the driving TFT is connected to a drain electrode of a switching TFT (not shown).
If the first electrode 28 and the second electrode 32 function as an anode electrode and a cathode electrode, respectively, the first electrode 28 may be formed of a transparent conductive material, such as indium tin oxide. The second electrode 32 may be a double layer including a metallic thin film, which has a low work function, that contacts the organic EL layer 30. The organic EL layer 30 is formed by an evaporation method using a low molecular material. The first electrode 28 is formed at each sub-pixel through a patterning process using a shadow mask.
The transmissive-type OELD device according to the relate art is difficult and inefficient to manufacture as a large display panel when the organic EL layer of low molecular material is formed by an evaporation method. In addition, the shadow mask process requires an additional processing apparatus that may damage the device. Further, the shadow mask process becomes more difficult as the resolution for a device is increased. In addition, the related art transmissive-type OELD device has poor transmissive characteristics due to lack of consideration for transmissive characteristics of array elements in a non-emissive area.